1. Field of the Invention
This invention relates to a new process for the production of powder lacquers based on hydroxy-functional copolymers and ketoxime-blocked, lacquer-grade polyisocyanates and to the use of the powder lacquers obtained by this process for the production of coatings on heat-resistant substrates.
2. Description of the Prior Art
Oxime-blocked polyisocyanates and their use in powder coatings are already known (see, for example, EP-A-432,257 (WO 91/00267) U.S. Pat. No. 3,857,818, DE-OS 2,200,342 or Japanese patent application 70-94 941, publication No. 75-27 057 (C.A. 84: 107 163 s)). Some of these publications also mention the combination of the oxime-blocked polyisocyanates with hydroxy-functional copolymers. For example, the last-mentioned Japanese document describes the combination of (i) xylylene diisocyanate wherein the isocyanate groups are partly blocked with a special oxime, with (ii) a hydroxy-functional polyacrylate. Example 4 of U.S. Pat. No. 3,857,818 describes the combination (i) of a high-melting, butanone-oxime-blocked isocyanate prepolymer based on 4,4'-diisocyanatodicyclohexyl methane with (ii) a hydroxy-functional polyacrylate which is solid at room temperature.
Powder lacquers of the type in question are attended in particular by the disadvantage that they can only be stoved at relatively high temperatures because, due to the high melting points of the individual components, poor flow properties are obtained at moderately elevated temperatures.
Another disadvantage of the powder lacquers in question is that they only result in brittle, inflexible lacquer films which are unsuitable, for example, for coating automobiles where tough, elastic and flexible coatings are required.
Accordingly, an object of the present invention is to provide new powder coating compositions which may be cured at comparatively low temperatures of 120.degree. to 160.degree. C., possess satisfactory flow at comparatively low temperatures and result in high-quality coatings.
This object may be achieved in accordance with the process of the present invention which is described in detail hereinafter. The choice of the binder components A) and B), the use of catalyst C) and the production of the powder coating compositions via the intermediate stage of a homogeneous solution of the individual constituents mentioned, are critical to the invention.